This sounds like it is the Capture recapture method. All you have to do is this:
So an example would be:
50/n = 10/400
From 10 to 50 you times by 5.
Therefore you do 400X5= 2000
So your answer is n=2000
Your questions:
1. 39/2 = n/9
So 9/2 is 4.5
So 39X4.5= 175.5
N=175.5
2. 3/n = 24/5.6
So 24/3=8
5.6/8= 0.7
N=0.7
3. n/9 = 24/6
So 9/6= 1.5
24X1.5=36
N=36